Capacitance sensing systems can sense electrical signals generated on electrodes that reflect changes in capacitance. Such changes in capacitance can indicate a touch event (i.e., the proximity of an object to particular electrodes). Electrical sense signals can be degraded by the presence of noise.
Noise in capacitance sensing systems can be conceptualized as including “internal” noise and “external noise”. Internal noise can be noise that can affect an entire system at the same time. Thus, internal noise can appear on all electrodes at the same time. That is, internal noise can be a “common” mode type noise with respect to the sensors (e.g., electrodes) of a system. Sources of internal noise can include, but are not limited to: sensor power supply noise (noise present on a power supply provided to the capacitance sensing circuit) and sensor power generation noise (noise arising from power generating circuits, such as charge pumps, that generate a higher magnitude voltage from a lower magnitude voltage).
In touch screen devices (i.e., devices having a display overlaid with a capacitance sensing network), a display can give rise to internal noise. This common mode type noise can be addressed by a common mode type filter that filters out noise common to all electrodes in a sense phase.
External noise, unlike internal noise, can arise from charge coupled by a sensed object (e.g., finger or stylus), and thus can be local to a touch area. Consequently, external noise is typically not common to all electrodes in a sense phase, but only to a sub-set of the electrodes proximate to a touch event. Sources of external noise can include, for example, charger noise or liquid crystal display (LCD) noise. Charger noise can arise from charger devices (e.g., battery chargers that plug into AC mains, or those that plug into automobile power supplies). Chargers operating from AC mains can often include a “flyback” transform that can create an unstable device ground with respect to “true” ground (earth ground). Consequently, if a user at earth ground touches a capacitance sense surface of a device while the device is connected to a charger, due to the varying device ground, a touch can inject charge at a touch location, creating a localized noise event. LCD noise is also external noise, but is additive common-mode noise, attributable to an LCD panel, used for example in a touch-screen display.
Other sources of external noise can arise from various other electrical fields that can couple to a human body, including but not limited to AC mains (e.g., 50/60 Hz line voltage), fluorescent lighting, brushed motors, arc welding, and cell phones or other radio frequency (RF) noise sources. Fields from these devices can be coupled to a human body, which can then be coupled to a capacitance sensing surface in a touch event. Thus, during sensing, a sense current can be generated in response to the sense event. However, at the same time, a noise current can arise, for example due to the operation of a charger. The noise current can be additive and subtractive to the sensed signal, and can give rise to erroneous sense events (i.e., touch indicated when no touch occurs) and/or erroneous non-sense events (i.e., touch not detected). While capacitance sensing systems can include common mode type filtering, such filtering typically does not address the adverse affects of external noise, as such noise is not present on all electrodes, but rather localized to electrodes proximate a sense event.